The following question was part of the Leaving Cert Higher Level examination paper in 2010:
There are many problems with these questions, worth 4.25% of the Higher Level paper. They are, of course, completely reliant on recall. There is no chemical understanding required here, and any student with a decent set of bullet-point notes will be able to rattle off that atoms are small, indivisible, and have identical atomic mass for a given element (8 marks) & Thomson, Rutherford, and Millikan (9 marks). 17 marks in the bag – no chemistry required.
Some easy marks on an exam are excusable. A much larger issue here is how the history of chemistry is presented and examined as a list of facts and dates. This is an awful pity, because the history of chemistry allows for a powerful illustration of how the scientific process works. Instead of isolated dates and facts as listed in the syllabus* and presented in textbooks**, we could instead present the history of chemistry as a continuing revision of our understanding based on new information as it comes to light. Much more important, and relevant, and let’s be honest, interesting, than the dates and facts themselves are the whys? and so-what? questions. Why did Thomson complete his experiments? In other words, instead of focussing on the events in isolation, what is more relevant are the links between them.
What does this mean in practice? How can this be approached at a level appropriate for this stage (and in the time available). My personal opinion is that the reasons and context of these experiments and the conclusions from the results are much more valuable than the names of the (white, male) scientists that completed them. Let’s take Thomson as an example. In his original paper, he says that his experiments are set against the backdrop of two competing theories: cathode rays are wave-like in nature or that cathode rays are charged particles. He writes:
The electrified-particle theory has for purposes of research a great advantage over the aetherial theory, since it is definite and its consequences can be predicted.
In other words – if the particles are charged, it is a lot easier to design an experiment to investigate their presence. Thomson did not, as popular textbooks and revision guides suggest, prove that these particles were negative.† This had already been done two years previously by the Frenchman Jean Perrin. Thomson writes of that experiment:
This has been proved to be the case by Perrln… [his] experiment proves that something charged with negative electricity is shot off from the cathode, travelling at right angles to it.
Thomson used a variation of Perrin’s experiment to incorporate both an electometer (to measure electric charge) and a phosphorescent screen (to measure path of cathode rays). He used a magnet to deflect the cathode rays through a narrow slit where the charge of the rays could be collected. His experiment thus demonstrated that the negative charge and the rays were one and the same. His surmised:
Thus this experiment shows that however we twist and deflect the cathode rays by magnetic forces, the negative electrification follows the same path as the rays, and that this negative electrification is indissolubly connected with the cathode rays.
The next question is whether these charges were ions, or a universal charge. Briefly, the charge to mass experiment shows that they were not ions, as the value was constant, and different ions would have different charges, hence giving a different ratio. In addition, as the charge to mass ratio was an enormous number, it must mean that the mass was very light indeed – much lighter than an atom. Thus the conclusion is that this is a universal particle, that is smaller than the atom.
This brief discussion of this single component in the history of chemistry argues against point-by-point facts that are out of context and offer little understanding. Indeed, over-simplification has led to a misrepresentation of the history of our understanding of this topic. A much greater learning opportunity exists by shifting the emphasis to consider the context of each experiment. This allows the student to consider the development of the idea of atomic structure from Dalton through Thomson in a much more holistic manner. Niaz and Rodríguez argue that the current emphasis of presenting these topics as ‘rhetoric of conclusions’ is not conducive to towards understanding the scientific process. They write:
It is not far fetched to suggest that such presentations lead the students to memorize the experimental details and ignore the underlying rationale for the experiment, which could facilitate conceptual understanding. Thus the theoretical rationale (‘heuristic principle’) in which the experiment is conducted is more important than the experiment itself.
†Thomson did complete an experiment using electrostatic plates, but this was an extension of previous experiments whereby he created a near vacuum to show that the rays were deflected by a small charge – something that hadn’t been proven.
- Niaz M. and Rodríguez M. A. (2000) Teaching Chemistry as Rhetoric of Conclusions or Heuristic Principles – A History and Philosophy of Science Perspective, Chemistry Education: Research and Practice in Europe, 1(3), 315-322.
- Thomson, J. J. (1897) Cathode rays, The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 44, 293-316.
*This is what the syllabus requires to be covered:
Very brief outline of the historical development of atomic theory (outline principles only; mathematical treatment not required):
Dalton: atomic theory; Crookes: vacuum tubes, cathode rays; Stoney: naming of the electron; Thomson: negative charge of the electron; e/m for electrons (experimental details not required); Millikan: magnitude of charge of electrons as shown by oil drop experiment (experimental details not required); Rutherford: discovery of the nucleus as shown by the α−particle scattering experiment; discovery of protons in nuclei of various atoms; Bohr: model of the atom; Chadwick: discovery of the neutron
**This is an outline of the coverage of Dalton to Rutherford in a popular textbook (my summary):
Beginning of nineteenth century: Dalton put forward atomic theory to explain results from his experiments with gases. Three assumptions of Dalton’s theory given (matter consists of atoms, atoms are indivisible, atoms cannot be created nor destroyed).
Mid nineteenth century: electric currents passing through gas at low pressure caused a glow. Crookes postulated the idea of cathode rays. These rays caused a paddle wheel to rotate away from cathode.
End of nineteenth century: Thomson investigated charge of these rays and ‘discovered’ the electron. He also used an electromagnet and determined the e/m ratio, whose number we don’t need to know. Thomson also proposed the plum pudding model.
Early 20th century: Millikan determined the charge and mass of the electron with the oil drop experiment. Rutherford experiment and observations led to a revised model of atom.